1. Field of the Invention
The present invention relates to a network device and a data control program that perform multicast transmission and multicast reception over a ring or cascade network.
2. Description of the Related Art
Multicast transmission has been used in a ring or cascade network, particularly, in a resilient packet network (RPR) having two way channels. Specifically, when a transmitter node transmits data over the network, receiver nodes, which has already stored therein a specific drop address that uniquely identifies data, receives data that is allocated with the specific drop address over the network. However, in multicast method, the data is transmitted to a large number of unspecified nodes, irrespective of whether the data is required by the nodes, resulting in generation of unnecessary packets and wasteful consumption of network bandwidth.
To explain the conventional technology with reference to FIG. 9, for example, devices A to H are connected over a ring RPR network having two-way transmission channels. Data MC1 is transmitted from the device A through multicast over the network in both directions and is received by the devices C, G, and H. The devices C, G, and H already have stored in them the address of the data MC1 in the form of a drop address, and drop the data MC1 circulating in the network based on the stored drop address. The data MC1 keeps circulating in the network, consuming network bandwidth, until it is deleted. Therefore, various technologies have been disclosed to prevent wasteful consumption of network bandwidth.
For example, Japanese Patent Application Laid-open No. 2003-249940 discloses a technology in which a transmitter node sends a receipt acknowledgement for the transmitted data. A receiver node, upon receiving the receipt acknowledgement, sets an acknowledgement bit concerning data that the receiver node has already received ON, and transmits the receipt acknowledgement to the next node downstream of itself. When the receipt acknowledgement returns to the transmitter node after having circulated the network once, the transmitting node checks whether data circulating in the network has been received by all nodes that are to receive the data based on the acknowledgement bits, and deletes, when it is determined that all the nodes have received the data, the multicast packet on data.
However, the conventional technology still proves inadequate in preventing wasteful consumption of network bandwidth. This is because the data packet is deleted only after circulated the entire network. It means that data is transmitted to even a node on the network that does not need the data.
Specifically, as shown in FIG. 9, the data MC1 is transmitted to all the devices A to H, whereas only the devices C, G, and H require the data MC1 in the conventional multicast method. That is, the data is wastefully transmitted to the devices D, E, and F (represented by dotted line), leading to wasteful consumption of network bandwidth.